1. Field of The Invention
The present invention relates to an improved electrophotographic photosensitive member having a protective layer comprising a specific metal oxide thin film, which is highly sensitive to electromagnetic waves such as light (which herein means in a broad sense those lights such as ultraviolets rays, visible rays, infrared rays, X-rays, and .gamma.-rays and which prevents occurrence of a filming and provides an improvement in the transfer efficiency and also in the utilization efficiency of a developer in the electrophotographic image-forming process, wherein high quality image reproduction is assurred. The present invention also relates to an electrophotographic apparatus such as a copying machine, a laser beam printer, or the like which comprises said electrophotographic photosensitive member, a charging means, an exposure means, a developing means and a cleaning means and in which image formation is conducted by way of an electrophotographic process. The present invention includes a process for the production of said electrophotographic photosensitive member.
2. Related Background Art
There have been provided a variety of photosensitive members for electrophotography each having a light receiving layer comprised of a non-single crystal material containing silicon atoms as a matrix such as an amorphous silicon material (hereinafter referred to as a-Si material). For simplification purpose, the photosensitive member for electrophotography will be hereinafter referred to as electrophotographic photosensitive member, and the photosensitive member having a light receiving layer comprised of an a-Si material will be hereinafter referred to as a-Si electrophotographic photosensitive member.
Such electrophotographic photosensitive members have been evaluated as being high in surface hardness, exhibiting a high sensitivity not only against visible light (for example, of from 400 to 700 nm) but also against a semiconductor laser beam of 770 to 800 nm without being deteriorated even upon repeated use over a long period of time, and their surface being capable of being maintained uniform even upon repeated use over a long period of time. In view of this, they have been desirably used as a device for recording, for example, in high speed copying apparatus or laser beam printers in which an electrophotographic technique is utilized.
Now, the foregoing non-single crystal material containing silicon atoms as a matrix used as the light receiving layer in the electrophotographic photosensitive member is generally formed by means of a plasma CVD process in which plasma discharge is caused in a gaseous atmosphere composed of, for example, silane gas, and if necessary, hydrogen gas, a dopant-imparting raw material gas, or other raw material gas to thereby decompose these gases whereby forming a film comprised of a silicon-containing non-single crystal material on a substrate made of aluminum for example. The reason why the plasma CVD process is often employed in the formation of the light receiving layer of the photosensitive member is that the plasma CVD process enables to uniformly form a film having a uniform thickness and a homogeneous property over the entire surface of a large area substrate such as a cylindrical substrate.
In the case of the a-Si electrophotographic photosensitive member, it is desired to have a surface protective layer in order to prevent occurrence of a unfocused image when image formation is conducted under high humidity environmental conditions. Such surface protective layer is usually composed of a silicon-containing material selected from the group consisting of SiC series materials, SiN series materials, and SiO series materials, a carbon material selected from the group consisting of amorphous carbon materials and diamond series carbon materials, or other material selected from the group consisting of BN series materials. The surface protective layer comprised of any of these materials is usually formed by means of the plasma CVD process. Specifically, U.S. Pat. No. 4,786,574 discloses electrophotographic photosensitive members having a surface protective layer comprised of a SiC series material. U.S. Pat. No. 4,965,154 discloses electrophotographic photosensitive members having a surface protective layer comprised of a SiN series material. U.S. Pat. No. 4,932,859 discloses electrophotographic photosensitive members having a surface protective layer comprised of an amorphous carbon material. Other than these, U.S. Pat. No. 5,153,086 and U.S. Pat. No. 5,183,719 disclose electrophotographic photosensitive members having a surface protective layer comprised of a metal oxide material. Further, U.S. Pat. No. 5,324,609 discloses electrophotographic photosensitive members having a surface protective layer comprised of an organic high molecular material.
By the way, in recent years, in order to improve the efficiency in the office works, there has been an increased demand for a further improvement in the rate of operation of the electrophotographic apparatus used, specifically, for a further prolongation of the lifetime of the electrophotographic photosensitive member used in the electrophotographic apparatus. The a-Si electrophotographic photosensitive member satisfies such demand since it has a sufficient surface hardness and semipermanently exhibits desirable electric characteristics and its life is inexhaustible in the ordinary use.
However, as for the electrophotographic apparatus provided with the a-Si electrophotographic photosensitive member, there is a disadvantages such that when electrophotographic image formation is continuously conducted over a long period of time, a film comprising a resin component of a developer is liable to form on the surface of the a-Si electrophotographic photosensitive member. In order to maintain the surface of the a-Si electrophotographic photosensitive member in a clean state, it is necessary to periodically conduct a maintenance work to remove such film formed on the surface of the a-Si electrophotographic photosensitive member by suspending the operation of the electrophotographic apparatus. Hence, the electrophotographic apparatus provided with the a-Si electrophotographic photosensitive member is not satisfactory in terms of the rate of operation.
In order to diminish the above maintenance work, there is considered a manner of improving the cleaning property of the developer at the surface of the a-Si electrophotographic photosensitive member. For this purpose, it will be effective to design the surface of the a-Si electrophotographic photosensitive member and the developer such that they can be well released one from the other, specifically such that the developer is hardly deposited on the surface of the a-Si electrophotographic member. In order to attain this situation, a manner in which the surface of the a-Si electrophotographic photosensitive member is designed to have an increased contact angle against water (that is, an increased water repellency in other words).
Herein, in general, the generally known a-Si photosensitive members are of a surface contact angle against water in the range of 20.degree. to 100.degree.. As for the a-Si photosensitive members which are practically usable as an electrophotographic photosensitive member in an electrophotographic apparatus, their surface contact angles against water are somewhat different one from the other depending upon their surface constituent materials. But they are mostly of a surface contact angle against water in the range of 50.degree. to 70.degree.. It is difficult for them to have a further increased water repellency.
Now, there are known fluororesins which impart a water repellency of providing a surface contact angle against water of near 180.degree. for layers formed by them. The present inventors prepared a plurality of a-Si electrophotographic photosensitive members each having a surface protective layer formed of one of those fluororesins. And their surfaces were examined with respect to water repellency and adhesion with the layer situated under the surface protective layer. As a result, it was found that they are sufficient in terms of the water repellency (that is, their surface contact angles against water are of near 180.degree.) but their surface protective layers are poor in adhesion. The present inventors conducted further examinations by repeatedly conducting electrophotographic image formation using a high speed copying machine provided with a blade cleaning means in which one of the a-Si electrophotographic photosensitive members was installed. As a result, it was found that any of the a-Si electrophotographic photosensitive members is liable to cause a removal between the surface protective layer and the layer situated thereunder and thus, any of the a-Si electrophotographic photosensitive members is not practically usable.
Separately, a so-called coating film comprising an inorganic material for a semiconductor device may be formed by means of a sputtering process other than the above described plasma CVD process. Other than these film-forming process, it may be formed by means of a manner in which a so-called sol-gel process is utilized.
In the case of forming a coating film using an inorganic material such as an amorphous material, e.g., glass, it is formed through an atmosphere maintained at a high temperature of higher than 1000.degree. C. or by using an expensive specific VPE (vapor phase epitaxy) apparatus.
In the case of forming a coating film composed of an inorganic material such as a silicon-containing amorphous material as a surface protective layer on a light receiving layer of an electrophotographic photosensitive member, it is usually formed by means of such plasma CVD process as previously described or a sputtering process because of relatively low heat resistance of the light receiving layer. However, in this case, the light receiving layer on which the surface protective layer is formed is required to be sufficiently resistant to a temperature of 100.degree. to 300.degree. C. under a given vacuum atmospheric condition. In view of this, such coating film composed of an inorganic material such as a silicon-containing amorphous material is employed only in an a-Si electrophotographic photosensitive member or a certain OPC electrophotographic photosensitive member in which a highly heat resistant binder is used.
Now, the above described sol-gel process makes it possible to form a functional material at a remarkably low substrate temperature. U.S. Pat. No. 5,153,086 and U.S. Pat. No. 5,183,719 disclose photoelectric photosensitive members prepared using the sol-gel process. Particularly, these U.S. patent documents disclose a manner of obtaining a sol solution by dissolving an alkoxide compound or an acetylacetonate as an organic metal complex in an alcohol or a mixture comprising an alcohol and water and subjecting the resultant to hydrolysis, dispersing electrically conductive metal oxide fine powder in the sol solution to obtain a dispersion, applying the dispersion onto a substrate for an electrophotographic photosensitive member or a light receiving layer formed on a substrate by a spray coating or dip coating manner, and subjecting the resultant to heat treatment to remove the solvent, whereby a hard thin film as an under coat layer or a surface protective layer is formed.
For the gelled thin film thus formed as the surface protective layer, it is still insufficient in terms of the adhesion with the photosensitive layer and the surface releasing property (that is, the surface water repellency). In addition to this, there is a problem in that a film comprising a resin component of a developer is liable to deposit on the surface of the surface protective layer.
The present inventors prepared a plurality of a-Si electrophotographic photosensitive members each having a surface protective layer formed by means of the above solgel process. And their surfaces were examined with respect to surface releasing property (surface water repellency) and adhesion with the layer situated under the surface protective layer. As a result, it was found that they are insufficient in terms of not only the surface water repellency but also the adhesion. Using a high speed copying machine provided with a blade cleaning means in which one of the a-Si electrophotographic photosensitive members was installed, the present inventors conducted further examinations by repeatedly conducting electrophotographic image formation over a long period of time. As a result, it was found that a removal is liable to occur between the surface protective layer and the light receiving layer situated thereunder and a film comprising a resin component of a developer is liable to deposit on the surface of the a-Si electrophotographic photosensitive member.
The deposition of such resin component film on the surface of the electrophotographic photosensitive member leads to providing copied images accompanied by defects, wherein a high quality copied image cannot be stably provided. In addition to this, it sometimes prevents a developer on the surface of the electrophotographic photosensitive member from sufficiently transferring onto a recording sheet such as a paper in the transfer step of the image formation process. The residual developer remained on the surface of the electrophotographic photosensitive member is removed by means of a cleaner such as a cleaning blade in the cleaning step. However, in the case where the residual developer is excessively present on the surface of the electrophotographic photosensitive member, it is difficult to be completely removed in the cleaning step, resulting in promoting the deposition of the foregoing resin component film on the surface of the electrophotographic photosensitive member. In the case where the transference of a developer on the surface of the electrophotographic photosensitive member to a recording sheet is insufficient particularly at a halftone portion, there entails a problem in that a image reproduced is accompanied by an uneveness in terms of the image density. This situation is remarkable in the case of conducting image formation using a high speed copying machine in which an electrophotographic photosensitive member is operated at a high speed, wherein the foregoing resin component film is more liable to deposit on the surface of the electrophotographic photosensitive member to increase the probability of causing the above problems. Further, the reduction in the amount of the developer dedicated for reproduction of an image results in increasing the amount of the developer to be removed by means of the cleaner. This situation leads to reducing the utilization efficiency of the developer. This problem should be solved also in view of reducing the amount of wastes.
The foregoing maintenance work which is periodically conducted for removing the resin component film deposited on the surface of the electrophotographic photosensitive member not only reduces the rate of operation of the electrophotographic apparatus but also increases an occasion of accidentally damage the surface of the electrophotographic photosensitive member upon cleaning it. Such damage occurred at the surface of the electrophotographic photosensitive member entails a serious problem in terms of the reuse.